1. Field of the Invention
The present invention relates to the production of cementitious structural materials, and is more particularly concerned with the production of lightweight gypsum wallboard.
2. Description of the Prior Art
Gypsum wallboard is manufactured commercially by processes which are capable of operation under continuous high speed conditions. In carrying out these processes, an aqueous slurry of calcined gypsum is continuously deposited on a moving sheet of paper which constitutes one cover sheet for the final product. The slurry so deposited is leveled and a second cover sheet simultaneously applied, one of the cover sheets being folded over the edges of the unset slurry. The gypsum forming the core between the opposed sheets is then allowed to set, and the resulting board is cut into panels of the desired length and passed through a drying kiln in which the excess water is removed and the gypsum brought to a final set. In the operation of high speed gypsum wallboard machines of the type described it is important that quick setting gypsum slurries are employed in order that production may be maintained at a suitable rate. Thus, as is well known, the calcined gypsum employed usually includes set accelerators so that the core of the board will be essentially completely set within a few minutes after the slurry has been formed.
In the commercial manufacture of gypsum wallboard it is generally desired that the product be light in weight, and this result has been accomplished by incorporating a pre-generated tenacious foam directly into the gypsum and water slurry at the time that it is initially mixed. This procedure is well known in the art and is disclosed in Roos U.S. Pat. No. 2,017,022, issued Oct. 8, 1935, and in Roos U.S. Pat. No. 2,080,009, issued May 11, 1937. In accordance with the disclosures in these patents, a pre-generated tenacious foam is admixed with the calcined gypsum and water under conditions to form a foamed slurry, and this slurry is then used for the manufacture of gypsum wallboard to produce a product having a lightweight cellular core.
In the manufacture of lightweight gypsum wallboard of the type described, the finished product is subjected to a drying operation in a kiln in order to remove any residual free water that may be left in the board. This kiln drying sometimes results in a weakening of the board, particularly at its edges, due to the fact that some dehydration of the gypsum dihydrate may occur adjacent to the edges of the board, as a result of the surface area exposed to the drying operation adjacent the edges. This phenomenon occurs even though the raw edges of the core are covered by fold-over portions of the cover sheets.
Many attempts have been made to strengthen the edges of wallboard which has been foamed to make it lightweight. In U.S. Pat. No. 2,985,219, dated May 23, 1961, a process and apparatus are disclosed for strengthening the edges of the wallboard produced by utilizing foaming. In carrying out this process, a foamed gypsum slurry is prepared by conventional techniques under relatively mild conditions of blending or mixing such that the foam is not broken to any substantial degree during the operation. Thereafter, at least a portion of the resulting slurry is intensely or violently mixed under conditions such that a substantial portion of the foam in the slurry is broken. During this supplemental mixing, additional foam is added to the mixer to replace at least a portion of the foam broken in the supplemental mixing operation. The foamed slurry discharged from the supplemental mixture is deposited on a moving support of a conventional plasterboard machine, whereafter it is formed by a roller and passed through conventional settings, cutting and drying zones. This results in a product which is lightweight, but yet has strengthened edges.